Varying angle of gas impingement in gas knife process for removing excess coating

ABSTRACT

IN THE GAS KNIFE PROCESS FOR CONTROLLING THE WEIGHT OF AN APPLIED COATING, THE ANGLE OF GAS IMPINGEMENT ON THE STRAND IS VARIED, BY EMPLOYING AT LEAST TWO THIN GASEOUS STREAMS WHICH ARE CAUSED TO IMPINGE UPON EACH OTHER, PRIOR TO STRIKING THE STRAND. THE RESULTANT JET (ANALOGOUS TO THE RESULTANT VECTOR OF SEVERAL FORCES ACTING ON A PARTICULAR BODY) IS THEN EMPLOYED TO ACT AS A GASEOUS BARRIER TO EXCESS LIQUID ON THE STRAND. THE ANGLE OF IMPINGEMENT OF THE RESULTANT JET IS EASILY MANIPULATED BY VARYING THE RATIO OF THE FLOW RATES OF THE INDIVIDUAL GASEOUS STREAMS,

R. C. MOYER VARYING ANGLE OF GAS IMPINGEMENT IN GAS KNIFE PROCESS FORREMOVING EXCESS COATING Filed Dec. 16. 1971 2 Sheets-Sheet 1 FIG.

FIG. 2.

60 PLENUM A PRESSURE WIRE TRA l/EL PRESSURE y 1973 R. c. MQYER 3,236,174

GLE OF GAS IMPINGEMENT IN GAS KNIFE VARYING AN PROCESS FOR REMOVINGEXCESS COATING Filed Dec. 16, 1971 2 Sheets-Sheet 2 United States PatentOfice Patented May 29, 1973 3,736,174 VARYING ANGLE OF GAS IMPINGEMENTIN GAS KNIFE PROCESS FOR REMOVING EX- CESS COATING Robert C. Moyer,Monroeville Boro, Pa., assignor to United States Steel Corporation FiledDec. 16, 1971, Ser. No. 208,709 Int. Cl. Bc 11/06; (323a N00 US. Cl.117-102 M 6 Claims ABSTRACT OF THE DISCLOSURE In the gas knife processfor controlling the weight of an applied coating, the angle of gasimpingement on the Strand is varied, by employing at least two thingaseous streams which are caused to impinge upon each other, prior tostriking the strand. The resultant jet (analogous to the resultantvector of several forces acting on a particular body) is then employedto act as a gaseous barrier to excess liquid on the strand. The angle ofimpingement of the resultant jet is easily manipulated by varying theratio of the diow rates of the individual gaseous streams.

This invention is related to a method for providing a continuouslyvariable impingement angle for use in the gas wiping of strand (wire,strip and sheet) materials.

Fluid wiping methods have long been employed for controlling the coatingweights of various liquids which are continuously applied to strandmaterials. Thus, for well over three decades gas knives have beenemployed in the paper industry to control coating weights at line Speedsof up to 1400 f.p.m. Analagous procedures have also been employed in theapplication of paints and lubricants and have now come to the forefrontin controlling the quality and quantity of the film applied in thehot-dip coating of metals. In employing these various gas knife methods,it is not only desirable to control the weight or thickness of thecoating, but the control of features such as uniformity, textureandbrightness is equally important. In attempting to control these variousfeatures which contribute to the overall qualityof the coating, it hasbeen found that many operating variables play an important role. Thus,such variables as gas flow rate, distance from nozzle to strand, strandspeed, strand temperature, liquid bath temperature and angle of gasimpingement may play a singificant role, depending on the particularcoating being applied. With respect to the angle of impingement,variation in the angle will not only change the extent of the wipingaction (i.e. thickness) but will also alter the degree of cooling andthe splatter pattern exerted by gas barrier. These latter effects beingespecially important in hot-dip metal coating applications. Thus, asdescribed in US. Pat. 3,459,587, a positive impingement angle has beenfound to be most effective for terne coating, while a negative angle ispreferred for galvanized coatings. Therefore, the ability toconveniently change the impingement angle from one coating operation toanother would be most desirable. Similarly, during any particularcoating procedure it is also desirable to be able to automatically varythe impingement angle to compensate for changes in any of the aboveenumerated variables, e.g. strand speed, bath temperature, etc.

Heretofore, the only manner by which such a continuously variableimpingement angle could be achieved was by use of rather cumbersomemechanical linkages for rotation of the nozzles. The instant method, onthe other hand, permits such variation to be achieved easily and with ahigh degree of control by merely varying the ratio of gas flow ratesthrough two or more convergently directed nozzles. It has now beenfound, that if at least two nozzle orifices are disposed at increasingdistances (along the length of the strand) from the liquid bath and thegaseous streams from these nozzles are directed at convergent angleswith respect to each other so that they impinge upon each other prior tostriking the strand, the streams will, in effect, fuse into oneresultant jet, Which in turn may be employed to impinge upon the strandat the desired angle.

The objects and advantages of the instant method will be betterunderstood by reference to the following detailed description, when readin conjunction with the appended claims and the accompanying drawings,in which:

FIG. 1 is a cross-sectional representation of an integral doubleorifice, variable angle gas knife for performing the method of theinstant invention;

FIG. 2 is a nomographic representation of the variation in angularcoverage which may be achieved by use of the device of FIG. 1; and

FIG. 3 is a cross-section of an integral triple orifice device useful inyet another embodiment of the invention; and

FIG. 4 is a cross-section of a further embodiment, showing the use of aseparate, double orifice system for use in the instant method.

The devices and methods described in 'FIGS. 1 to 3 are specificallyadopted for use in the gas wiping of strand materials of circularcross-section, such as wire or tubing. However, it will be apparent tothose skilled in the art, the manner in which similar devices may beemployed for the wiping of strand materials of rectangular crosssection,such as sheet and strip.

FIG. 1 is a cross-sectional illustration of a device for wire wiping inwhich two convergent nozzle orifices are fixed in relation to each otherin an integral, generally cylindrical apparatus. In this device, a60-degree conical Wiping range is achieved through the use of convergingannular streams 2 and 3 which fuse into resultant jet 4. Streams 2 and 3are formed respectively by orifices 5 and 6 supplied through independentplenums 7 and 8 in which the Working pressures may be varied. Thepressures within plenum 7 (for control of the 60 jet) and plenum 8 (forcontrol of the 0 jet) will directly affect the velocity of gaseousstreams 2 and 3, thereby controlling both the velocity and angle of theresultant wiping jet 4.

. While variation in the ratio of pressures will generally be sufiicientto provide the required control of the angle of the resultant wipingjet, further control can be obtained by varying the throat widths oforifices 5 and 6. Thus, to adjust the throat of the 60 degree orifice,top component 9 is merely screwed up or down until the desired width isobtained. Similarly, base component 10 may be turned to achieve thedesired throat width of the 0 orifice. Lock rings (not shown) may bethen be employed to secure both the top and base components in place.

FIG. 2 illustrates the wipe-angle pressure relationship of the doubleorifice device shown above. With no pressure in 60 degree plenum 7 and aworking pressure of 20 ounces per square inch in plenum 8 feeding the 0degree orifice, the wiping jet will issue perpendicular to the wire.Under normal operation, with both orifice throat widths equal, and thepressure varied within a constant total amount, (in this case 20 oz./in.the complete 60 degree wiping range can be covered. The device may beoperated to require no more gas than a conventional, single orificeunit, since the total of the throat widths of the two orifices need notbe greater than that of a conventional single orifice.

A three orifice, three-plenum wipe design which provides a degree wipingrange 30 to +60") is shown in FIG. 3. For purposes of this invention,wipe angles are described with respect to the horizontal, therefore,gaseous streams co-current with the direction"of"" strand travel areconsidered negative and those countercurrent to strand travel arepositive. Thus, referring to FIG. 3, orifices 12 and 13 are inclined at+60 degrees and +30 degrees respectively, and orifice 14 is inclined at30 degrees. Analogous to the device of FIG. 1, the gasous streams whichcombine to form the resultant wiping jet are formed by orifices 12, 13and 14, which are in turn supplied through independent plenums 15, 16and 17, respectively. Orifice openings 12 and 14 are adjusted by turningtop component 18 and base component 19 respectively. Opening 13 isadjusted by turning adjusting ring 20; one thread being left handed andthe other right handed. Thus, depending on the direction of turning thewalls which define opening 13, will either separate or come together.

FIG. 4 presents a schematic illustration of a further embodiment, withdiscrete nozzles, i.e., wherein the individual plenums are not containedwithin an integral wiping device. Here, for example, the plenumpressures in the nozzles 21 and 22 need not be equal, thereby providingthe further capability of simultaneously achieving both difierentimpingement angles and different gaseous flow rates on opposing sides ofthe strip. This feature is especially desirable for achieving theincongruent overlap discussed more fully in US. Pat. 3,459,587 and/ orfor the production of strip with diiferential coating weights onopposing sides.

The instant method provides a degree of versatility and control whichcan greatly enhance the capability of a process employing the gas knifeprinciple. It provides a convenient method for close control of gasimpingement angle which may easily be adapted to automatic control.Merely varying the impingement angle will not affect a correspondingchange in the effective distance between orifice mouth and the coatedstrand, and thereby, undesirably introduce-a further variable in thecoating process. It is adaptable for use with any of the wellknownwiping gases (e.g. air, stream, inert gas) and may be employed, equallyas well, for systems in which the strand emerges either horizontally orvertically from the coating bath.

I claim:

1. In the gas knife process for controlling the film applied from thedip coating of a strand of material through a liquid bath, an improvedmethod for varying the angle of gas impingement on said strand, whichcomprises:

projecting gaseous streams at converging angles with "respect toeachothe'r fromat least'two nozzle orifices;

disposed at increasing distances from said liquid bath,

whereby said streams are caused to interact with one another and therebycombine into a resultant jet, which in turn-is caused .to impinge uponsaid strand at the desired angle and act as a gaseous barrier to controlthe thickness and quality of saidliquid film, the variation in the angleof the resultant jet-being achieved by-varying the ratio of the flowrates of each of said gaseous streams. 5 2. The method of claim1,'wherein the nozzle orifice closest to said bath is inclined at anangle of -30 degrees and the orifice furthest from said bath is inclinedat an angle of degrees to achieve an angular Wiping range of degrees. 11

3. The method of claim 1, wherein said strand. is or circularcross-section, and said gaseous streams define a conical pattern. V

4. The method of claim 1, wherein said strand is of rectangularcross-section and said gaseous streams define a substantially planarpattern. 1 i p M 5. The. method of claim 4, wherein the nozzles orificesdisposed on opposite sides of said strand are substantially equidistantfrom said bath and are fed from independent plenums.

6. The method of claim 1, wherein said liquid bath is molten metal.

References Cited I STATES PATENTS Kurokawa .1 17,- 102 M ALFRED L.- LEAV ITT, Primary isxtimilier i J. R. BATTEN, JR.,'Assistant Examiner

